It would certainly be worth asking at the farm about it. It's definitely round and it's definitely round for a reason.
I love spotting these. They usually denote raths or ringforts in ireland and they are quite common here. However, I have spotted one or two that don't seem to be. Half circular detours in field wals are my fave. There's one in the middle of the four stone circles at Grange near Cong in Mayo ... a fifth circle? could be.
There is another in the Dargle valley in co. Dublin that I need to scramble down and look at, which has huge boulders forming the walls.
Yours has to be worth checking out with the locals. It looks great.